WO 2010063576 discloses a device for jetting droplets of a fluid at a high temperature, wherein the fluid is actuated by generating a Lorentz force in the fluid, further referred to as Lorentz actuation. To be able to generate a Lorentz force the fluid must comprise an electrically-conductive fluid. The device is suited to eject droplets of fluid at a high temperature, in particular of a molten metal or a molten semi-conductor, more in particular of metals having a high melting temperature (e.g. higher than about 1200 K), such as gold, silver, copper, titanium and the like. A Lorentz force is generated in the fluid, by applying an electric current pulse through the fluid, the fluid being positioned in a magnetic field. A direction and magnitude of the resulting force is related to the cross product of the electric current and the magnetic field vector: =×.
The Lorentz actuation method uses current pulses, which due to the Joule effect additionally heat the fluid and eventually the jetting device. The generated heat (Q [W]) is proportional to the square of the applied current (I [A]) and the total resistance (R [Ω]) of the parts of the print head through which the actuation current runs, comprising the electrode resistance, the print head material resistance, the liquid metal resistance, and contact resistances (e.g. contacts between electrodes and print head material, contact of electrodes with the liquid metal). The generated heat is further proportional to the duration of the current pulse (Δt [s]) and the pulse frequency (f [Hz]): Q˜I2*R*Δt*f (as derived from Joule's first law). For the purpose of jetting droplets according to the above described method, the applied current may be very high (i.e. in the order of 100 A-200 A). At very low frequency (e.g. ˜1-10 Hz) and short pulse widths (e.g. <50 μs) the Joule effect is small.
However, the intended operating regime of the jetting device is at high frequencies (e.g. about 5 kHz or even higher). It is observed that at such high frequencies the average temperature of the jetting device, in particular of the nozzle can get extremely high and therefore lead to overheating of the jetting device. This situation is undesired because it affects the jetting process, because the properties of molten metals and semi-conductors are temperature dependent. Moreover, overheating may cause damage to the receiving substrate, because the temperature of the droplets reaching the substrate may become too high. Eventually, the jetting device may become damaged. For example because of softening or even melting of the materials making up the jetting device (such as the electrodes), in particular when materials having a high melting temperature (e.g. above 1000° C.) are jetted.
Therefore a need exists for a method for adequately controlling the temperature and preventing overheating of a jetting device for jetting droplets of a fluid at a high temperature, wherein the fluid is actuated by generating a Lorentz force in the fluid.
In addition there may be other situations requiring a method for adequately controlling the temperature of a jetting device for jetting droplets of a fluid at a high temperature, such as:                situations wherein strongly heated substrates for printing are used. A strongly heated substrate (e.g. at 500° C.) may influence the temperature of the nozzle;        situations wherein various actuation pulses are used in the jetting-printing process. Various actuation pulses may result in a fluctuation of the nozzle temperature;        situations wherein all fluid present in a fluid chamber arranged in the jetting device to hold the fluid (also termed cartridge) has to be jetted. In such a situation the temperature of the nozzle may significantly decrease (e.g. with more than 100° C.), when the fluid level in the fluid chamber decreases, which may need to be compensated for by adequate temperature control;        situations wherein instabilities of the heating source occur.        
It is therefore an object of the present invention to provide such a method.
It is another object of the present invention to provide a jetting device suitable for performing such a method.